Hydraulics Field Observations

Project Overview

Visit a local waterway and make observations of hydraulic processes
Annotate oblique photographs of a channel, indicating flow magnitude and direction
Estimate discharge of a channel using minimal instrumentation
Find an example of a hydraulic jump and describe locations of subcritical and supercritical flow

Instructions for this assignment can be found here.

Location Map

Field Sketch

Date: 02/08/2025

Time: 12:30-1:00PM

Conditions: Overcast, beginning to lightly snow. Water level is likely close to base flow since this is the first storm in a few weeks.

Aerial field sketch of channel with labeled hydraulic features.

Annotated Photographs of Field Site

Uniform Flow

On this section of the Logan River, large boulders that protruding from the surface of the water cause frequent flow separations near the south bank (right side in this photograph). Uniform flow can be observed on the north side of the channel (left side of the photograph) where boulders do not interrupt the flow. The water in this part of the channel flows with similar turbulence, magnitude, and velocity.

Flow Seam and Reattachment Zone

A flow seam exists at this site between the slower- and faster-moving sections of the channel. A shear zone is present where the water moving at different velocities and magnitudes recombine. The reattachment zone can be seen past the shear zone as the water rejoins the path of uniform flow on the north side of the channel.

Convergent and Divergent Flow

Zooming in, it is clear that the combination of convergent and divergent flow patterns create an eddy within the observed shear zone. Boulders on the south side of the channel cause both divergent flows (left side of the photograph) and convergent flows (right side of the photograph) in close proximity. As these flows recombine, an eddy is formed from flows spiraling inwards on themselves.

Observed Surface Flow Types

Boulders primarily control surface flow types in this reach. On the south side of the channel, lines of boulders create pools where water velocity and magnitude is very low. Wakes over large substrate on the channel bed create rapids on the north side of the channel. Areas of uniform flow in the center of the channel that are not interrupted by boulders create riffles due to turbulent flow.

Discharge Estimation

Field sketch of channel cross-section.

Water velocity was much more variable along the south bank on this section of the Logan River, where emergent boulders constantly interrupted flow. This made it difficult to estimate discharge within 1.5 m of the south bank. Therefore, discharge was estimated by dropping an orange peel into the area of uniform flow, which was consistently around 0.5 m in depth and made up most of the channel width (3.5 m).

A rough estimate of discharge was found by taking the cross-sectional area of the channel in the uniform flow section multiplied by the flow velocity. The flow velocity was found by timing how long it took the orange peel (10.23 sec) to travel 10 m downstream after being dropped in the center of the uniform flow section. The discharge estimate was calculated by the following:

Q = V*A --> Q = (10 m/10.23 sec)*(0.5 m * 3.5 m) = 1.7 m3/s

This is an underestimate of actual discharge from this channel section because the 0.5 m of the channel width was left out of the analysis.